The present invention relates to an evaporator which can be advantageously applied in Multi Effect Distillation (MED) processes.
Multi Effect Distillation (MED) processes have been used in industry for juice evaporation, to concentrate a substance, production of salts and for salty and marine water distillation for fresh water production, for desalination of seawater, brackish waters and in general any water or liquid containing dissolved solids, in order to produce fresh water. In a MED process, only a portion of a liquid feed submitted to the heat transfer surfaces is evaporated. Each stage of the MED process, also called “effect” operates at a different pressure. The remaining liquid of each effect, usually called brine, is fed to the liquid inlet of a next effect, where again a part thereof is evaporated into vapour. Produced vapour in one effect is also passed to the next effect and will give up heat to boil the remaining liquid transferred to the next effect due to temperature differences between them. Examples of MED configurations are shown in U.S. Pat. Nos. 3,245,883, 3,884,767, 3,261,766, 3,021,265.
In order to transfer heat, shell and tube type exchangers, plate type exchangers or tube bundles with sprays nozzles are applied. Frequently these exchangers are made from metal corrosion resistant materials like titanium and the like. Known disadvantages of these designs based on metals are their sensitivity to fouling and scaling and that they are only economical attractive at large scale.
New types of evaporators (or condensors) have been developed to account for these disadvantages. These new types apply plastics like polypropylene, poly vinyl chloride and poly ethylene to reduce material cost and prevent scaling. The non-scaling behaviour of plastics is known throughout the industry. However, plastics are less heat conductive compared to metals which results in the requirement to separate the fluids by a very thin wall in order to be of any practical use.
GB 1,157,301 has disclosed an evaporator for evaporation which applies plastic tubes, made of flexible polyethylene film, with a falling evaporating liquid film on the outside and condensing vapour on the inside of the tubes. It is also mentioned that stand pipes are required in order to facilitate the necessary supporting.
CN 101012071 has disclosed also a falling film evaporator utilizing plastic film for the construction of the flow path for the condensing vapour. The flow paths are connected according to a ‘mattress’-like configuration on which the liquid is distributed in order to create a liquid film.
An alternative plastic ‘mattress’ design is described by T. N. Scheffler and A. J. Leao in ‘Fabrication of polymer film heat transfer elements for energy efficient multi-effect distillation’. The latter reference also describes the main disadvantage of plastic falling film evaporators, which is the non-wetting feature of plastics. When the plastic heat transfer surface is not sufficiently wetted by the respective liquid, then the effective heat transfer area will be much less. Also the heat transfer coefficient will be lower because of the non-wetting features of plastic and the relative low velocity of the falling liquid film.
Another disadvantage of the known plastic thin wall evaporators is, that not every flow path of the first fluid (vapour) is surrounded by the flow path of the other fluid (vapour-liquid) to the maximum extent. The known ‘mattress’-like configuration results therefore in a lower effective heat transfer area per volume and will also lead to stability and strength problems when the tubes of the mattress have several meters of length. Finally, spray nozzles or other liquid film distributors could plug rapidly when the evaporating liquid contains solids and/or scaling fragments.
It is obvious that the designs discussed above and their assembling processes are complicated, cumbersome, laborious, time-consuming and therefore expensive, offering a suboptimal final product with respect to its final heat transfer properties.